Known examples of Hsp90 family proteins include Hsp90α proteins, Hsp90β proteins, grp94, and Hsp75/TRAP1 (“Pharmacology & Therapeutics”, 1998, vol. 79, p. 129-168, and “Molecular Endocrinology”, 1999, vol. 13, p. 1435-1448).
Hitherto, benzoquinone ansamycin antibiotics, such as Geldanamycin and Herbimycin, and Radicicol are known as compounds that bind to Hsp90 family proteins (for example, see Non-Patent Documents 1 and 2). It is reported that these compounds all bind to Hsp90 family proteins and inhibit the functions of Hsp90 family proteins, thereby exhibiting pharmacological activities such as anti-tumor activity. Therefore, compounds that bind to Hsp90 family proteins are possibly useful as therapeutic agents for diseases associated with Hsp90 family proteins or proteins to which Hsp90 family proteins are bound (Hsp90 client proteins).
Geldanamycin derivatives (for example, see Non-Patent Document 3) and Radicicol derivatives (for example, see Non-Patent Documents 4 to 6) are reported to exhibit antitumor effects.

Novobiocin and PU3 are reported to bind to Hsp90 family proteins (for example, see Non-Patent Document 7 and 8).
Reblastatin, Autolytimycin, and 17-O-Demethylreblastatin are known as benzenoid ansamycin derivatives. Reblastatin is reported to inhibit retinoblastoma (Rb) protein phosphorylation to arrest the cell cycle at the G1 phase (for example, see Patent Document 1 and Non-Patent Document 9). Autolytimycin and 17-O-Demethylreblastatin are reported as anti-HSV-1 agents (for example, Non-Patent Document 10) and Oncostatin M inhibitors (for example, see Non-Patent Document 11). Furthermore, Compound A and derivatives thereof are reported to exhibit Hsp90 inhibitory activity and cytostatic activity (see Patent Document 2). Compound B (see Non-Patent Document 12) and Compound C (see Patent Document 3) are known. Furthermore, Compound D (see Non-Patent Document 13) and Compound E (see Non-Patent Document 14) are known.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 9-286779
[Patent Document 2] U.S. Published Application No. 2005/0026894
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 1-175970
[Non-Patent Document 1] “Cell Stress & Chaperones”, 1998, vol. 3, p. 100-108
[Non-Patent Document 2] “Journal of Medicinal Chemistry”, 1999, vol. 42, p. 260-266
[Non-Patent Document 3] “Investigational New Drugs”, 1999, vol. 17, p. 361-373
[Non-Patent Document 4] “Cancer Research”, 1999, vol. 59, p. 2931-2938
[Non-Patent Document 5] “Blood”, 2000, vol. 96, p. 2284-2291
[Non-Patent Document 6] “Cancer Chemotherapy and Pharmacology”, 2001, vol. 48, p. 435-445
[Non-Patent Document 7] “Journal of National Cancer Institute”, 2000, vol. 92, p. 242-248
[Non-Patent Document 8] “Chemistry & Biology”, 2001, vol. 8, p. 289-299
[Non-Patent Document 9] “Journal of Antibiotics”, 2000, vol. 53, p. 1310-1312
[Non-Patent Document 10] “Chinese Chemistry Letters”, 2001, vol. 12, p. 903-906
[Non-Patent Document 11] “Journal of Antibiotics”, 2000, vol. 53, p. 657-663
[Non-Patent Document 12] “Journal of Medicinal Chemistry”, 1995, vol. 38, p. 3806-3812
[Non-Patent Document 13] “Tetrahedron Letters”, 1991, vol. 42, p. 6015-6018
[Non-Patent Document 14] “Journal of Organic Chemistry”, 2003, vol. 68, p. 8162-8169